First, it is important to give some information about the Lotus-eaters (Lotophagoi in Greek). Odysseus encounter the Lotus-eaters on his journey homeward after Troy.
When a north wind drove them off course, they landed on the island of this group of people (book 9).
The local inhabitants of the land had an interesting practice of eating a Lotus plant, which made them forgetful (blissfully forgetful). The men who had eaten had to be dragged back to their ships and even chained to row!
The visit to the Lotus-eaters adds to the book in a dramatic way, because on a long journey the easiest thing to do is to forget and never to make it back. This is an important point, because at times Odysseus lingers in various places. Think of his time with Calypso. The point is that Odysseus may forget his past life, wife, and throne in Ithaca. The dramatic question is: "Will Odysseus remember and be faithful?"
The phrase “to eat lotus” is used figuratively to mean “to forget."